Heat Exchanger Having Improved Drain System

ABSTRACT

A heat recovery steam generator (HRSG) (A) has a casing ( 2 ) and several heat exchangers in the casing. Some of those heat exchangers ( 12,16 ) take the form of coils ( 18 ) having multiple lower headers ( 36 ) into which the lower ends of tubes ( 18 ) open. The tubes ( 18 ) and headers ( 36 ) hold or may hold water and need to be drained from time to time. To this end, the heat ex changer ( 12,16 ) has a drain system ( 20 ) provided with drain pipes ( 40 ) that are connected to the lower headers ( 36 ) and contain check valves ( 48 ) that permit the water to flow away from the headers ( 36 ), but not into them. The drain pipes ( 40 ) lead to and open into a drain manifold ( 46 ). The drain pipes ( 40 ), check valves ( 48 ) and drain manifold ( 46 ) are assembled in a shop along with the coil ( 18 ) and, when the HRSG (A) is assembled in the field, lie within the interior of the casing ( 2 ). The drain system ( 20 ) also includes a single common drain line ( 52 ) that extends downwardly from the drain manifold ( 46 ) and through the floor ( 8 ) of the casing ( 2 ), beyond which it is provided with a drain valve ( 56 ) that, when opened, allows water to drain from the coil ( 18 ).

RELATED APPLICATION

This application derives priority from and otherwise claims the benefitof U.S. provisional application 61/346742 filed May 20, 2010, which isincorporated herein by reference.

TECHNICAL FIELD

This invention relates in general to heat exchangers and moreparticularly to a heat exchanger having an improved drain system, to thedrain system itself; and to a heat recovery steam generator containingthe heat exchanger.

BACKGROUND ART

The gas turbines that power electrical generators discharge exhaustgases at extremely high temperatures. Heat recovery steam generators(HRSGs) extract the heat from the gases to produce steam that powerssteam turbines that in turn drive more electrical generators.

The typical HRSG includes multiple heat exchangers located one after theother in the flow of hot exhaust gases from a gas turbine. Among heatexchangers are an economizer for elevating the temperature of feedwater, an evaporator for converting the high temperature waterdischarged by the economizer into saturated steam, and a superheater forconverting the saturated steam into superheated steam. Many HRSGs havemore than one economizer, evaporator, and superheater operating atdifferent pressures.

An economizer operates with a full charge of water. Indeed, it simplyheats the water so that the evaporator to which it is connected consumesless heat converting the water to saturated steam. The superheatercontains only steam during operation of the HRSG, but when the HRSG istaken off line and shut down, the steam within the superheater condensesand water collects in its lower regions. From time to time, economizersand superheaters require servicing, and many service procedures involvedraining the water that remains in those components. Likewise, someHRSGs operate in environments that experience temperatures below thefreezing temperature of water. When those HRSGs are taken out ofservice, the economizers and superheaters should be drained to preventwater from freezing in them. Hence, the economizers and superheatershave drain systems at their lower regions for removing water from them.

Every HRSG includes a duct-like structure, called a casing, having aninlet and an outlet. Hot gas from a gas turbine or other source entersthe casing at the inlet and flows through the casing, discharging at theoutlet. Within the casing the gas encounters at least one superheater,at least one evaporator, and an economizer, generally in that order withrespect to the flow of the gas. While some of the components of thedrain systems for both the economizer and the superheater exist withinthe casing, the remaining components lie outside the casing.

Economizers and superheaters include coils that can consist of multiplerows of tubes, with the tube rows being arranged one after the other inthe flow of hot gases (FIG. 2). Within each row of tubes the tubes areconnected to a lower header that extends horizontally and alsotransversely with respect to the gas flow. The lower headers facilitatecirculation through the coil and enable it to be drained. Drain pipingcannot be connected directly between headers because water will transferfrom header to header without passing through the tubes. This bypassfrom tube row to tube row to row will diminish the coil performance. Buteach coil requires drain piping that includes numerous small bore pipesand fittings, both internal of the HRSG casing and external to it aswell. The latter require penetration seals in the floor of the casing.

The typical coil (FIG. 2) has a drain pipe leading away from each lowerheader, through the casing to a valve located outside of the casing.Beyond its valve, each drain pipe opens into a drain manifold alsolocated external to the casing, and it contains a common drain valve toprovide redundancy. This requires multiple penetrations of the casing,and each penetration requires a seal that is welded to the pipe and tothe floor of the casing. It also requires making welds within theconfines of the casing where each drain pipe connects with its lowerheader. More welds are required externally of the casing at couplingsand elbows in the drain pipes as well as at the valve in each drainpipe. And of course still more welds are required to join each drainpipe to the drain manifold into which it opens. All of this work takesplace in the field where the HRSG is assembled, not in the shop wherethe coil is manufactured, and much of it requires workman to performtheir tasks in extremely confined spaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a heat recovery steamgenerator having heat exchangers constructed in accordance with andembodying the present invention;

FIG. 2 is a perspective view of a drain system for a prior art heatexchanger;

FIG. 3 is a perspective view of a coil for the heat exchanger of thepresent invention;

FIG. 4 is a sectional view of the coil shown in FIG. 3;

FIG. 5 is a perspective view of the drain system for the heat exchangerof the present invention; and

FIG. 6 is another perspective view of the drain system for the heatexchanger.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring now to the drawings (FIG. 1), a heat recovery steam generator(HRSG) A includes a casing 2, which is essentially a large duct havingan inlet 4 and an outlet 6. Between the inlet 4 and the outlet 6 thecasing 2 has a floor 8 that supports several heat exchangers forconverting subcooled water into superheated steam. A hot gas, typicallythe exhaust of a gas- or oil-fired turbine, enters the casing 2 at itsinlet 4, passes through the several heat exchangers which extract heatfrom it, and leaves through the outlet 6 at a substantially reducedtemperature. Among the heat exchangers are an economizer 12 for bringingsubcooled water, which might be the condensate from a steam turbine, toa higher temperature, an evaporator 14 for converting the heatedsubcooled water derived from the economizer 12 to saturated steam, and asuperheater 16 for converting the saturated steam to superheated steam.The HRSG may have more than one economizer 12, evaporator 14 andsuperheater 16 organized in groups that operate at different pressures.Each group has a pump located before or after its economizer 12, andthat pump controls the pressures at which the group operates. In termsof the flow of gas through the casing 2, for any group the superheater16 precedes the evaporator 14 and the evaporator 14 precedes theeconomizer 12. The economizer 12 and the superheater 16 will retainwater when the HRSG

A is not in operation, and this water can interfere with maintenanceprocedures. Moreover, if the HRSG A is subjected to freezingtemperatures, the retained water can freeze and damage the economizer 12and superheater 16.

Using the economizer 12 as an example, it includes (FIGS. 3 & 4) a coil18 that is located in the casing 2 and a drain system 20 that is locatedbelow the coil 18 and for the most part inside the casing 2. The coil 18has tubes 22 that extend vertically and are organized in multiple rows24 that extend transversely with respect to the flow of gas through thecasing 2. Above the tubes 22 the coil 18 has a supply header 26 thatextends over the row 24 of tubes 22 located downstream in the flow ofthe exhaust gas through the coil 18 and is provided with an inlet port28. At the upstream rows 24 of tubes, a discharge header 30 extends overthe tubes 22, and it has an outlet port 32. The supply header 26connects with and opens into the upper end of every other tube 22 of theendmost downstream row 24. The remaining tubes 22 of the endmostdownstream row 24 connect with the upper ends of adjacent tubes 22, thatis to say with every other tube 22, in the next row 24 through U-shapedtransitions 34. The upper ends of the remaining tubes 22 in the next roware connected at their upper ends to the upper ends of adjacent tubes 22in the next row 24 upstream from it through still more transitions 34.The same pattern of transitions 34 at the upper ends of the tubes 22continues through the coil 18 to the discharge header 30. There theupper end of every other tube 22 in the next to endmost row 24 and theupper ends of all of the tubes 22 in the endmost row 24 open into thedischarge header 30. The lower ends of the tubes 22 open into lowerheaders 36 that extend transversely with respect to the flow of gas,there being a separate lower header 36 for each row 24, except at theupstream end where a single lower header 36 services the two endmostupstream rows 24. Each lower header 36 has a drain port 38.

Subcooled water, such as the condensate from a steam turbine, enters thecoil 18 at the inlet port 28 of its supply header 26. The header 28distributes the water to every other tube 23 of the endmost downstreamrow 24 through which it flows downwardly into the lower header 36 towhich those tubes 22 are connected. The water rises out of the lowerheader 36 at the remaining tubes 22 of the endmost row 24. The waterflows into every other tube 22 of the next row 24 through the U-shapedtransitions 34 that connect those tubes 22. The water circulates throughthe coil 18 from one row 24 to the next in a like manner and at theendmost upstream rows enters the discharge header 30. It leaves theheader 30 at the outlet port 32 and flows on to the evaporator 14. Asthe water circulates through the coil 18 its temperature increases, buteven so it leaves the discharge port 32 in a subcooled condition.

The lower headers 36 lie above the floor 8 of the casing 2, while thedrain system 20 for the most part occupies the space between the lowerheaders 36 and the floor 8. The drain system 20 includes (FIGS. 5 & 6)drain pipes 40 that extend downwardly from the discharge ports 38, towhich they are connected, and then transition horizontally. Thus, eachdrain pipe 40 has a generally upright segment 42 and a generallyhorizontal segment 44. The discharger ports 38 are located near the endsof their respective headers 36, with the ports 38 for alternatingheaders 36 being located at opposite ends of those headers 36. Thus, thehorizontal segments 44 of the drain pipes 40 for alternating headers 36approach the bottom center of the coil 18 from opposite directions. Herethey open into a common drain manifold 46 that extends horizontallybeneath the headers 36. The drain pipes 40 are provided with checkvalves 48 that are oriented such that water will flow through the drainpipes 40 to the manifold 46, but not in the opposite direction. Thecheck valves 48, which may be located in either the upright segments 42or the horizontal segments 44, allow water to drain from the coil 18 andlower headers 36 when the manifold 46 is opened, but prevent water fromcirculating from header 36 to header 36 through the drain system 20 andthus bypassing the tubes 22. Actually, all of the drain pipes 40 shouldcontain a check valve 48, except the drain pipe 40 that connects withthe endmost header 36 at the downstream end of the coil 18. That drainpipe 40 need not contain a check valve 48, although it may. Each checkvalve 48 should open under a low head, that is to say with a smallpressure differential across it. Preferably it should take the form of aswing-type valve. If a check valve 48 does not close completely, onlyminor bypassing from header 36 to header 36 will occur.

The drain manifold 48 has a common drain port 50 that opens downwardly.All that resides above it, including the tubes 22, the headers 26, 30,36 as well as the drain pipes 40, the check valves 48 and the drainmanifold 46, may be assembled in a shop and shipped as a unit to thesite where the HRSG A is assembled. This includes most of the drainsystem 20. Contrast this with a conventional heat exchanger where thedrains and their valves are installed at the site of the HRSG atconsiderable expense and inconvenience.

Once the coil 18 is set onto the floor 8 of the casing 2, at the sitewhere the HRSG is assembled, a few additional procedures complete thedrain system 20. To this end, the drain port 50 of the drain manifold 46is connected to a common drain line 52 that extends downwardly from themanifold 46 and penetrates the floor 8 of the casing 2 at a single seal54. Below the casing 2 the drain line 52 contains two drain valves 56arranged in succession, with the last of the two providing redundancy.This eliminates the congestion of piping found below the floors ofcasings for HRSGs equipped with traditional drain systems.

While the coil 18 with the improved drain system 20 serves as theeconomizer 12 in the HRSG A, the coil 18 and its drain system 20 may beused on other heat exchangers having multiple lower headers 36 in whichwater may collect. For example, the superheater 16, while in operationcontains only steam, but when taken out of operation, that steam maycondense and occupy the lower regions of its tubes 22 and lower headers36. The superheater 16 may take the form of the coil 18, although withfewer tubes 22 and lower headers 36, and may have the improved drainsystem 20. Also, a feedwater heater, to the extent that it may differfrom an economizer, could be furnished with the drain system 20.

1. A heat exchanger comprising: tubes arranged generally vertically;lower headers into which the lower ends of the tubes open; drain pipesconnected to the lower headers and extending downwardly from the lowerheaders; and check valves in at least some of the drain pipes where theyare oriented to allow flow out of the lower headers, but notsubstantially into the lower headers.
 2. A heat exchanger according toclaim 1 and further comprising a drain manifold located below the lowerheaders, the drain pipes being connected beyond their check valves tothe drain manifold.
 3. A heat exchanger according to claim 2 whereineach drain pipe has a generally upright segment that is connected to oneof the lower headers and a generally horizontal segment that isconnected to the drain manifold.
 4. A heat exchanger according to claim3 wherein the lower headers are generally parallel, and the horizontalsegments of the drain pipes are generally parallel to the lower headers;and wherein the drain manifold extends generally transversely withrespect to the lower headers.
 5. A heat exchanger according to claim 4wherein the drain pipes connect with the lower headers beyond the drainmanifold, with the drain pipes for adjacent headers being on oppositesides of the drain manifold.
 6. A heat exchanger according to claim 4wherein the tubes are organized in rows, with the lower ends of thetubes for at least some of the rows opening into separate lower header.7. A heat exchanger according to claim 6 wherein the upper ends of someof the tubes in one row that is endmost open into a supply header; andwherein the tubes of another row that is endmost at their upper endsopen into a discharge headers.
 8. A heat exchanger according to claim 4and further comprising: a common drain line connected to the drainmanifold and extending downwardly from it, and a drain valve in thecommon drain line.
 9. A heat exchanger according to claim 8 and furthercomprising water in the lower headers, the drain pipes, the drainmanifold and the common drain line ahead of the drain valve.
 10. A heatrecovery steam generator comprising: a casing having an inlet and anoutlet and also a floor between the inlet and outlet; and the heatexchanger of claim 1 located in the casing with its drain pipes andcheck valves being located above the floor.
 11. A heat recovery steamgenerator comprising: a casing having an inlet and an outlet and also afloor between the inlet and outlet; and the heat exchanger of claim 4located in the casing with its drain pipes and check valves and drainmanifold being located above the floor; a common drain line connected tothe drain manifold and extending downwardly through the floor of thecasing; and a drain valve located in the common drain line and beingoutside of the casing.
 12. A heat recovery steam generator according toclaim 11 wherein the drain pipes connect with the lower headers beyondthe drain manifold, with the drain pipes for adjacent headers being onopposite sides of the drain manifold.
 13. A heat exchanger comprising: acoil including: tubes extending vertically and organized in rows; andlower headers into which the tubes at their lower ends open; and a drainsystem including; drain pipes connected to the lower headers andextending downwardly from them; a drain manifold into which the drainpipes open; check valves in at least some of the drain pipes andoriented to allow flow from the headers to the drain manifold but not inthe opposite direction; a common drain line extending downwardly fromthe drain manifold; and a valve in the common drain line.
 14. A heatexchanger according to claim 13 wherein the tubes of the rows open intoa separate lower header.
 15. A heat exchanger according to claim 13wherein each drain pipe includes an upright segment and a horizontalsegment.
 16. A heat exchanger according to claim 15 wherein the drainmanifold extends transversely with respect to the lower headers; whereinthe drain pipes open into the drain manifold at their horizontalsegments; and wherein the drain pipes are connected to the lower headersat their upright segments on opposite sides of the drain manifold, withthe upright segments of the drain pipes for adjacent lower headers beingon opposite sides of the drain manifold.
 17. A drain system for a heatexchanger having lower headers arranged side by side, said drain systemcomprising: drain pipes extending downwardly from the lower headers andthen generally horizontally; a drain manifold into which the drain pipesopen, the drain manifold extending generally transversely with respectto the drain pipes; check valves in the drain pipes and being orientedto allow fluid to flow out of the headers into the drain manifold, butnot in the opposite direction into the headers; a drain line extendingdownwardly from the drain manifold; and a drain valve in the drain line.18. A drain system according to claim 17 wherein the drain pipes areconnected to the headers near the ends of the headers, and the drainpipes for adjacent headers are at opposite ends of those headers.